Corrosion inhibitor composition for magnesium or magnesium alloys

ABSTRACT

The present invention relates to novel corrosion inhibitor compositions for magnesium or magnesium alloys and to a process for inhibiting the corrosion of such metals using such compositions. The corrosion inhibitor composition comprises a compound comprising a carboxyl group preventing the re-deposition of noble impurities which significantly decreases the corrosion rate by complexing said noble impurities, e.g. iron, nickel and copper.

FIELD OF THE INVENTION

The present invention relates to novel corrosion inhibitor compositions for magnesium or magnesium alloys and to a process for inhibiting the corrosion of such metals using such compositions.

BACKGROUND OF THE INVENTION

Magnesium is most lightweight of all structural metals, weighing 35 percent less than aluminum and 78 percent less than steel. Lightweight characteristics, wide availability and processability make magnesium alloys suitable for production of motor vehicle components, electric products, aircraft components, etc. Generally, magnesium and magnesium alloys are made into shaped articles mainly by die casting, extrusion or rolling. However, the percentage of magnesium alloys used in automobiles, electric products, aircraft components, etc. has been traditionally low. The reasons for the limited use of magnesium alloys are associated with the intrinsic properties of this family of alloys: low creep and corrosion resistance.

The corrosion resistance of magnesium or magnesium alloys depends on similar factors that are critical to other metals. However, because of the electrochemical activity of magnesium, the relative importance of some factors is greatly amplified. When unalloyed magnesium is exposed to air at room temperature, a gray oxide forms on its surface. Moisture converts this oxide to magnesium hydroxide, which is stable in the basic range of pH values, but not in the neutral or acid ranges.

For providing anti-corrosion properties, magnesium or magnesium alloys are generally treated with chromates. The chromate treatment nevertheless involves the difficulty in setting the conditions for the treatment, so that it has been desired to provide more convenient corrosion inhibiting processes. Furthermore, the chromate treatment has the drawback that when conducted, the treatment discolors the surface of the metal, depriving the metal of its luster. Furthermore, chromium compounds are rather toxic and harmful to the environment. Thus, processes are highly desirable which are less likely to burden the environment.

For achieving corrosion protection, magnesium can also be coated in an assortment of ways depending on the type of alloy used, the desired qualities of the finished material and the application in which it will be used. For example, magnesium can be coated with organic layers. These layers prevent against corrosion of the magnesium by insulating it from the outside environment.

Advanced coating systems also possess active corrosion protection that implies continuous corrosion protection, even in the event of local damage to the coating. This is achieved by incorporating corrosion inhibitors in the coating systems.

U.S. Pat. No. 6,569,264 B1 discloses a corrosion inhibitor composition for magnesium or magnesium alloys for use in protective coatings, which contains as an effective component, a phosphate, at least one compound selected from among aromatic carboxylic acids or salts thereof and a pyrazole or triazole. Published European patent application 1 683 894 A1 discloses the use of 1,2,3-triazoles, 1,2,4-triazoles or pyrazole as useful corrosion inhibitors for magnesium and magnesium alloys, which could be incorporated in protective coatings.

However, there is still the need to provide corrosion inhibitors for use in coatings for magnesium or magnesium alloys which have an improved effectiveness.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide novel corrosion inhibitor compositions for magnesium or magnesium alloys useful in coatings which are more effective and of lesser environmental concern than known corrosion inhibitor compositions and to a process for inhibiting the corrosion of such metals using such compositions.

This object is achieved by an article made of magnesium or a magnesium alloy coated with a coating composition comprising at least one corrosion inhibiting compound of Formula (1):

wherein R⁴ is one or more substituents selected from the group consisting of hydrogen; hydroxy; alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from hydroxy and carboxy; carboxy; —SO₃H and —NH₂, and salts thereof. Preferred compounds include salicylic acid, sulfosalicylic acid, methyl salicylic acid, aminosalicylic acid and salts thereof. The R⁴ group(s) may be located in ortho-, meta- and/or para-position relative to the carboxy group. Exemplary compounds include salicylic acid, 3-methylsalicylic acid, 4-methylsalicylic acid, 5-methylsalicylic acid, 6-methylsalicylic acid, 3-sulfosalicylic acid, 4-sulfosalicylic acid, 5-sulfosalicylic acid, 6-sulfosalicylic acid, 3-aminosalicylic acid, 4-aminosalicylic acid, 5-aminosalicylic acid, 6-aminosalicylic acid, 3,5-dinitro salicylic acid and salts thereof. Especially preferred are salicylic acid, 4-aminosalicylic acid, 5-aminosalicylic acid, 5-methylsalicylic acid, 3,5-dinitro salicylic acid and salts thereof.

Preferably the salts of compounds of Formula (1) are selected from their alkali metal or earth alkaline metal salts, more preferably the salts of compounds of Formula (1) are selected from their lithium, sodium or potassium salts, most preferably sodium salts are used.

Without wishing to be bound to any theory for patenting purposes, the inventors attribute the excellent corrosion inhibiting effect of compounds of Formula (1) to their capability of reducing the re-deposition of noble impurities. The present inventors found that noble impurities like, iron, copper and nickel, although being the sites of cathodic reaction, get detached from the corroding magnesium by undermining mechanisms and dissolve by forming Fe(II), Fe(III), Cu(I), Cu(II) and Ni(II) ions. Subsequently, these ions are being reduced and re-deposit on the surface of the magnesium or magnesium alloy. This enlarges the area of cathodic activity and accelerates corrosion. Thus, based on this finding, it has been found that prevention of re-deposition of noble impurities significantly decreases the corrosion rate. Re-deposition of dissolved iron, nickel and copper is effectively avoided by chemically binding said ions by means of complexing agents among which carboxylic acids and hydroxy carboxylic acids are excellent ligands for metal ions including iron, copper and nickel.

It is further preferred that the corrosion inhibiting compounds of Formula (1) form stable complexes with Fe(II) or Fe(III) or Cu(I) or Cu(II) or Ni(II) thereby preventing re-deposition of said metals onto the magnesium or magnesium alloy thereby significantly decreasing the corrosion rate. It is preferred that the corrosion inhibiting compounds form complexes with at least one of Fe(II), Fe(III), Cu(I), Cu(II) and Ni(II) ions with a stability constant of log K 3.5, wherein K is the stability constant of the formed complexes.

According to a preferred embodiment, the corrosion inhibiting compounds of Formula (1) are incorporated in a coating deposited on magnesium or a magnesium alloy. It is further preferred that the corrosion inhibiting compounds of Formula (1) are present in cavities of porous nano- or micro-particles distributed within the coating. Preferably, the porous nano- or microparticles are selected from the group consisting of zeolites, layered double hydroxides and silicas.

According to a preferred embodiment, the corrosion inhibiting compounds of Formula (1) are incorporated in micro- and nano-pores produced on the surface of magnesium or magnesium alloys. According to a further preferred embodiment, the corrosion inhibiting compounds of Formula (1) are incorporated in nano- and micro-particles distributed in micro- and nano-pores produced on the surface of magnesium or magnesium alloys. Preferably, the micro- and nano-pores are produced by an anodizing process, such as, for example, PEO (plasma electrolytic oxidation), MAO (microarc oxidation), anodization or spark anodization. Preferably, the PEO, MAO or anodic layer has a thickness of 2 to 50 μm.

The object of the present invention is further achieved by a process for inhibiting the corrosion of magnesium or magnesium alloys comprising the steps of a) providing magnesium or a magnesium alloy and b) coating the magnesium or magnesium alloy with a corrosion inhibiting coating comprising a corrosion inhibitor of Formula (1) or a salt thereof.

It is preferred that the process for inhibiting the corrosion of magnesium or magnesium alloys further comprises a step a1) between step a) and b), wherein in step a1) the magnesium or magnesium alloy is pre-treated with the novel corrosion inhibitor composition. In this way, metal impurities are dissolved from the surface of the magnesium or magnesium alloy before coating thereby further increasing the corrosion resistance.

The corrosion inhibitor composition is preferably used as a coating for magnesium or a magnesium alloy.

EXAMPLES

Preferred embodiments of the present invention are further illustrated by the following, non-limiting examples by referring to the figures below. Magnesium materials used for hydrogen evolution measurements presented in FIGS. 1 to 8 were as specified in Table 1. The ingots of HP Mg 51, WE43, ZE41, E21, AZ31, AZ91 and AM50 were shaved to receive the stripes with the surface area of 240 to 480 cm²/g. This was done to ensure the identical chemical composition of each portion of the alloy used for testing solutions of different inhibitors. The plates (5.0 cm²/g) of commercial purity magnesium (CP Mg 220) were tested.

TABLE 1 Noble impurities found* in the materials used for hydrogen evolution tests. Impurity, ppm Material Fe Cu Ni High Purity Mg (HP Mg 51 <1 <2 51) Commercial Purity Mg 220 5 <2 (CP Mg 220) WE43 38 47 46 ZE41 15 19 8 Elektron 21 (E21) 12 20 52 AZ31 15 14 3 AZ91 22 48 <2 AM50 8 13 3 *Analysed by spark emission spectroscopy

FIG. 1 shows the results (normalized values) of hydrogen evolution measurements during immersion of CP Mg (commercial purity Mg containing Fe—220 ppm) in 0.5% NaCl containing 0.05 M of salicylic acid (sodium salt), 5-sulfosalicylic acid (sodium salt), 4-aminosalicylic acid (sodium salt), 5-methylsalicylic acid (sodium salt), 3,5-dinitrosalicylic acid (sodium salt), 3-methylsalicylic acid (sodium salt), 1,2,4-triazole (comparative), benzotriazole (comparative) and NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was 5.6 to 6.9). Concentration of 3,5 dinitrosalicylic acid sodium salt was 0.002M. Concentration of all other inhibitors was 0.05M.

FIG. 2 shows the inhibiting efficiency (%) calculated from the data of FIG. 1 at 20 hours of immersion. Concentration of 3,5 Dinitrosalicylic acid was 0.002M. Concentration of all other inhibitors was 0.05M.

FIG. 3 shows the results (normalized values) of hydrogen evolution measurements during immersion of magnesium alloy WE43(Fe—38 ppm) in 0.5% NaCl containing 0.05 M of salicylic acid (sodium salt), 4-methylsalicylic acid (sodium salt), 4-aminosalicylic acid (sodium salt), 5-methylsalicylic acid (sodium salt), 3,5-dinitrosalicylic acid (sodium salt), 3-methylsalicylic acid (sodium salt), 5-aminosalicylic acid (sodium salt) 1,2,4-triazole (comparative), benzotriazole (comparative) and NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was 5.6 to 6.9). Concentration of 3,5-dinitrosalicylic acid sodium salt was 0.002M. Concentration of 5-methylsalicylic acid sodium salt was 0.03M. Concentration of all other inhibitors was 0.05M.

FIG. 4 shows the results (normalized values) of hydrogen evolution measurements during immersion of magnesium alloy AZ91(Fe—22 ppm) in 0.5% NaCl containing 0.05 M of salicylic acid (sodium salt), 4-methylsalicylic acid (sodium salt), 4-aminosalicylic acid (sodium salt), 5-methylsalicylic acid (sodium salt), 3,5-dinitrosalicylic acid (sodium salt), 3-methylsalicylic acid (sodium salt), 5-aminosalicylic acid (sodium salt) 1,2,4-triazole (comparative), benzotriazole (comparative) and NaCl (comparative); pH of resulting solutions of sodium salts (adjusted by NaOH) was 5.6 to 6.9). Concentration of 3,5-dinitrosalicylic acid sodium salt was 0.002M. Concentration of all other inhibitors was 0.05M.

In the examples shown in FIGS. 1 to 4, the corrosion of the magnesium or magnesium alloy in solution was determined by measuring the amount of evolving hydrogen because hydrogen is formed upon oxidation (corrosion) of Mg to Mg²⁺ at the surface of the magnesium or magnesium alloy. The more magnesium is oxidized to Mg²⁺ during corrosion, the more hydrogen is formed.

As can be seen in FIGS. 1 to 4, the novel corrosion inhibiting compounds efficiently protect the magnesium and magnesium alloys from corrosion and show significantly improved corrosion inhibiting efficiencies compared to 1,2,4-triazole and benzotriazole as known from U.S. Pat. No. 6,569,264 B1.

As it is shown in FIG. 2, salicylic acid shows a corrosion inhibiting efficiency of more than 60% better than benzotriazole and about 55% better than 1,2,4-triazole known from the prior art. Further improvements are achieved when 4-aminosalicylic acid, 5-aminosalicylic acid, 5-methylsalicylic acid, 3,5-dinitro salicylic acid and salts thereof are used as corrosion inhibitors. The values of the inhibiting efficiency (IF) were calculated using the following equation:

${IE} = {{\frac{{CR}_{0} - {CR}_{inh}}{{CR}_{0}} \cdot 100}{\%.}}$

were CR₀ is the corrosion rate in pure 0.5% NaCl and CR_(inh) is the corrosion rate in the presence of NaCl and inhibitor. The corrosion rate was determined as amount of H₂ (ml) evolved at 20 hours of immersion (one mole of evolved hydrogen is equal to one mole of dissolved magnesium).

The inhibitors are active in a wide range of concentrations, as demonstrated in FIGS. 3 and 4.

As it is evident from FIG. 4, the corrosion inhibiting effect of the novel corrosion inhibiting compounds is not restricted to a specific magnesium alloy, but present for a large variety of different magnesium alloys, e.g. HP Mg 51, CP Mg 220, WE43, ZE41, E21, AZ31, AZ91 or AM50. Independently of the magnesium alloy, the novel corrosion inhibiting compounds show a significantly improved corrosion inhibiting effect compared to 1,2,4-triazole. 

1. An article made of magnesium or a magnesium alloy coated with a coating composition comprising at least one corrosion inhibiting compound of Formula (1):

wherein R⁴ is one or more substituents selected from the group consisting of hydroxy; alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from hydroxy and carboxy; carboxy; —SO₃H and —NH₂, and salts thereof; or the coating composition comprises 3,5-dinitro salicylic acid or salts thereof as at least one corrosion inhibiting compound.
 2. The article of claim 1, wherein the compound of Formula (1) is selected from the group consisting of 3-methylsalicylic acid, 4-methylsalicylic acid, 5-methylsalicylic acid, 6-methylsalicylic acid, 3-sulfosalicylic acid, 4-sulfosalicylic acid, 5-sulfosalicylic acid, 6-sulfosalicylic acid, 3-aminosalicylic acid, 4-aminosalicylic acid, 5-aminosalicylic acid, 6-aminosalicylic acid, 3,5-dinitro salicylic acid and salts thereof.
 3. The article of claim 2, wherein the compound of Formula (1) is selected from the group consisting of 4-aminosalicylic acid, 5-aminosalicylic acid, 5-methylsalicylic acid, 3,5-dinitro salicylic acid and salts thereof
 4. The article of claim 1, wherein the salts of compounds of Formula (1) are selected from their alkali metal or earth alkaline metal salts.
 5. The article of claim 1, wherein the corrosion inhibiting compound is present in cavities of porous nano- or microparticles distributed within the coating.
 6. The article of claim 1, wherein the corrosion inhibiting compound is present in micro- and nano-pores produced on the surface of magnesium or magnesium alloys.
 7. The article of claim 6, wherein the corrosion inhibiting compounds are incorporated in nano- and microparticles distributed in micro- and nano-pores produced on the surface of magnesium or magnesium alloys.
 8. A method for inhibiting the corrosion of magnesium or magnesium alloys comprising the steps of: a) providing magnesium or a magnesium alloy and b) coating the magnesium or magnesium alloy with a corrosion inhibiting coating comprising at least one corrosion inhibiting compound of Formula (1):

wherein R⁴ is one or more substituents selected from the group consisting of hydroxy; alkyl, wherein the alkyl group is optionally substituted with one or more substituents selected from hydroxy and carboxy; carboxy; —SO₃H and −NH₂, and salts thereof; or the coating composition comprises 3,5-dinitro salicylic acid or salts thereof as at least one corrosion inhibiting compound.
 9. The method of claim 8, further comprising a step a1) between steps a) and b), wherein in step a1) the magnesium or magnesium alloy is pretreated with the corrosion inhibiting composition. 